The spontaneous formation of crystalline structures from conductive metallic surfaces is now a well-known problem where metal-coated electronic devices are concerned.
Such growth may occur from a wide range of metallic surfaces. Tin is a metal which is commonly used as a plating on electronic devices, and the crystalline structures which grow spontaneously from such tin plating have become known informally as “tin whiskers”.
The main problem arising from tin whiskers and similar crystalline structures is that they can reach considerable lengths, with some whiskers having been reported to be over 10 mm long. Tin whiskers can therefore easily bridge the small gaps between components and other conducting parts of an electrical device, thereby causing a short circuit. Once the short circuit has occurred, if the current conducted by the whisker is sufficiently large the tin whiskers are likely to melt or evaporate, so that often the cause of the occurrence is not clear. In the case of small currents, up to a few milliamps, the whiskers can often continue to produce shorting. This can lead to permanent failure of electrical devices with potentially devastating consequences. Larger currents can cause melting of the tin and sometimes the production of a tin vapour which can facilitate the passage of a current as an arc.
For example, the Galaxy IV satellite loss has been attributed to tin whisker growth. Other significant failures include car electronics failures and heart pacemaker failures.
Even if complete failure of the device does not occur, tin whisker growth can easily compromise the reliability, longevity and functionality of electrical devices.
Conformal coatings are often applied to electronic devices to protect against moisture, dust, chemicals and extreme temperatures. Ordinary conformal coatings, commonly used to coat electronic substrates, do not provide a sufficient deterrent to tin whisker growth. Conformal coatings are not designed to prevent whisker growth, but for other purposes such as to prevent dirt and moisture from contacting the electronics in the device.
Previously, the stiffness of coatings has been increased in an attempt to prevent or retard tin whisker growth. The stiffness of coatings has been increased by either alteration of the coating chemistry (for example by using a more highly cross-linked polymer which not only increases the stiffness but also increases the chemical resistance of the coating), or by the introduction of filler particles into the coating composition to create a composite. The former is not satisfactory because the coating becomes much more difficult to remove from the substrate, which is not desirable for devices which need to be inspected or repaired (re-worked). Conformal coatings often need to be removed in order to inspect, adjust or clean the device underneath, and may be transparent so that the underlying substrate or device can be seen.
Attempts have therefore been made to mitigate tin whisker growth by the application of a modified conformal coating including filler particles to the surface of the conducting metal.
US 2013/0171405 describes a method of obstructing metal whisker growth that includes providing a conductive structure comprised of a whisker forming metal, and forming a composite coating on the whisker forming metal. The composite coating may include a matrix phase of a polymer and a dispersed phase of reinforcing particles. The reinforcing particles are intended to provide the coating with enhanced hardness, peel force or rupture force.
WO 2007/143644 describes a conformal coating including a polymer matrix with hard or soft particles dispersed therein. Whiskers which penetrate the coating are deflected by the particles to reduce further penetration into, and protrusion through, the coating.
Similarly, WO 2008/067293 and US 2008/0216704 describe coatings including particles which act to deflect tin whiskers which penetrate the coating, or provide a “tortuous path” for the tin whiskers to reduce the likelihood that they will grow far enough to break the coating surface.
Tin Whisker Growth
Tin whiskers are metallic crystalline tin structures which emerge from the surface of tin metal and grow outwards from the surface in an elongated protrusion. They grow in a number of forms including filamentary, nodular and odd-shaped eruptions. FIG. 1 shows SEM images of the surface of tin with a multitude of tin whiskers clearly visible.
As noted above, the problems caused by tin whisker growth can be very serious. Tin whiskers can grow to considerable lengths and bridge gaps between two different parts of a structure, even where such gaps are relatively large. When such bridging occurs between two conducting parts of an electrical circuit, a short circuit arises with potentially serious consequences. Even where a short circuit does not occur, the performance of the device is likely to be compromised. Whiskers may become detached causing shorting events in other areas of the electronic device. Whiskers can vapourise with the passage of larger electrical currents. The tin metal vapour can promote serious high current arcing. Finally, whiskers can cause distortion of electronic signals due to an ‘aerial’ effect.
Previous Attempts at Tin Whisker Growth Mitigation
Ordinary conformal coatings commonly used to coat electronic substrates do not provide a complete mitigation of tin whisker growth. Conformal coatings are not designed to prevent whisker growth, but for other purposes such as to prevent dirt and moisture from contacting the device. FIGS. 2 and 3 show tin whiskers penetrating standard conformal coatings and continuing to grow away from the surface as normal.
Previously, the stiffness of coatings has been increased in an attempt to prevent or retard tin whisker growth. FIG. 4 shows a tin whisker flattened by interaction with a stiff coating. The stiffness of coatings has been increased by either alteration of the coating chemistry, or by the introduction of particulates into the coating composition to create a composite. The former is not satisfactory because the coating becomes much more difficult to remove from the substrate, which is not desirable for devices which need to be inspected or repaired.
FIG. 5(a) shows the penetration of a tin whisker through a low-stiffness coating. The coating presents almost no protection against the growth of tin whiskers.
FIG. 5(b) shows the effect on tin whiskers when a coating of intermediate stiffness is applied to a metallic surface. “Tenting” of the coating occurs as can be seen from the SEM image, before eventual penetration of the whisker through the coating. Further growth of the whisker may then occur. The overall effect of the coating is simply delaying whisker growth.
FIG. 5(c) shows a high stiffness coating applied to a metal surface. No tenting or whisker penetration occurs. However, the coating is not easily removed from the surface which is a significant disadvantage when access to the surface is required after the coating has been applied.
None of the methods outlined in the documents discussed previously are certain to prevent the growth of tin whiskers. Rather, they provide coatings which deflect or slow down the growth, or simply provide a coating with enhanced stiffness. There is, therefore, a need for enhanced coating compositions and conformal coatings which more directly address the problem of tin whisker growth and provide effective mitigation of the growth of all kinds of metallic crystalline structures. Such coatings could more fully address the problems described above. There is also a need for coatings which offer effective mitigation of metallic crystalline structure growth, but retain desirable properties of workability, removability and/or transparency.